The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Receivers, such as portable geographic positioning receivers, commonly use signals from satellites to obtain a “position fix,” or an estimate of the current geographic location. Generally speaking, a receiver calculates the position fix by analyzing the time multiple satellite signals take to reach the receiver from known locations. In the systems available today, there must be a clear line of sight between the receiver and at least four satellites at the time of positioning to obtain an accurate position fix. A clear line of sight refers to a path from the satellite to receiver void of any obstacles that may completely block the satellite signal. Four satellites are needed to estimate four unknowns: the three-dimensional position of the receiver and the time offset of the receiver clock.
Often, objects such as trees occlude satellite signals from the receiver. Such occlusions should prevent the calculation of a position fix, but in practice a receiver may still receive a signal from an occluded satellite after the signal reflects off surrounding obstacles. In these situations, the receiver often obtains an erroneous position fix. Moreover, even when a receiver has a line of sight to four or more satellites, atmospheric factors such as temperature, pressure, and humidity sometimes affect signal propagation so as to introduce error in calculating a position fix.